A radioactive nucleus undergoing beta decay emits ionising radiation in the form of a beta particle, which is a high-speed electron or positron. Beta emitters are used in medical diagnosis and treatment and in industrial thickness gauges and are also formed as fission products from nuclear reactions. The ionising nature of beta radiation presents a potential health hazard in that it can cause serious human tissue damage, especially from within the body. As such, radioactive contamination monitors are important for monitoring contamination of personnel who may be exposed to radiation.
Radioactive contamination monitors can employ a variety of detector technologies, including scintillation detectors, solid-state detectors, and gaseous detectors. A scintillation detector comprises a scintillator whose fluorescence, when excited by ionising radiation, is measured using a photomultiplier tube.
In solid-state detectors, the ionizing radiation interacts with the semiconductor material and excites electrons out of the valence band and into the conduction band. An applied electric field causes a measurable net migration of the electrons and holes left behind.
Gaseous detectors include Geiger-Müller (GM) tubes, ionisation chambers, sealed-gas proportional counters and gas flow proportional counters. Gaseous detectors measure the ionisation of a fill gas (also called a counting gas) by ionising radiation, by generating an electric field across the fill gas and measuring the electric output resulting from the ionisation event.
A GM tube operates with a large voltage across the fill gas, between the cathode tube wall and a central anode. The fill gas comprises an inert gas, such as helium, neon or argon. Any ionising radiation entering the tube ionises the fill gas and the resulting ions and electrons are accelerated to the cathode and anode, respectively. The electrons gain sufficient kinetic energy to cause further ionisation and the resulting electron avalanche produces a large current pulse at the anode. The pulse is amplified and detected, but there is no information in the amplitude or shape of the pulse about the type of radiation which caused the pulse; the pulse is the same whatever the type of ionising radiation causing the pulse (i.e. regardless of the number of original ion pairs produced by the ionizing radiation). Levels of radiation are measured by the number of pulses counted (with background count rates measured and subtracted as appropriate).
By contrast, an ionisation chamber typically has a relatively lower voltage applied between its electrodes. As such, individual ions and electrons produced by ionizing radiation travel to their respective electrode, but there is no multiplication of ion pairs or avalanche. The relatively low speed of the ions and electrons is such that one ionization event overlaps with the next, and the drift of the ions constitutes an electric current (perhaps as low as 10−15 A), which is amplified and measured. Again, it is not possible to distinguish between the different types of ionizing radiation.
Between these detector types lies the proportional counter. In a proportional counter, the electric field strength set up between the electrodes is higher than in an ionization chamber, so that electron avalanches may be produced. However, the electric field strength is not as high as for a GM tube, so that the gas multiplication is more controlled. The proportional counter relies on gas multiplication to augment the number of electrons produced by the initial interactions of the ionising radiation with the fill gas. In the presence of the electric field, the free electrons will migrate towards the wire anode. The wire is very fine, typically of around 50 μm diameter, so the electric field strength close to the wire is very large. Electrons within a given radius from the anode, typically of around 100 μm, are accelerated to kinetic energies greater than the ionization potential of the fill gas molecules, so that further ionization of the fill gas takes place. The creation of secondary and further ion pairs from a primary ion pair as the electrons travels closer to the anode in this way is called a Townsend avalanche.
The important point with a proportional counter is that the gas multiplication (i.e., the factor by which a single primary ion pair increases the number of free electrons due to its avalanche) is substantially constant under given operational parameters. As such, the size of the charge pulse at the anode is proportional to the number of initial ionization events caused by the radiation. Alpha particles deposit significantly more energy in the fill gas than beta particles, so cause more ionization and therefore larger pulses in the detector. The pulse size can accordingly provide a measure of the type of ionizing radiation and of the energy imparted to the fill gas by it.
Sealed-gas proportional counters contain a sealed fill gas, typically either xenon or krypton, and have a relatively thick detector window, for sealing purposes (such as titanium, at 5-6 mg/cm2). Gas flow proportional counters operate with a continuous flow of fill gas through the detector, typically either argon and methane or argon and carbon dioxide, and generally have a relatively thin detector window (such as aluminized boPET (biaxially-oriented polyethylene terephthalate), available under the name, Mylar®, at around 1 mg/cm2).
One problem with the above types of detector is that they cannot distinguish between beta particles and gamma radiation. This is because gamma radiation can generally produce a similar degree of ionization as beta radiation, either directly in the fill gas or by interacting with the material of the detector chamber itself and producing an energetic electron, resulting in a similar detector output.
In addition, sealed-gas proportional counters tend to have a limited lifetime, because microscopic leaks can lead to contamination of their fill gas. Also, the entrance window generally needs to be thick, to contain the gas, and, as a consequence, the window can cut out low-energy beta radiation. A further disadvantage is their cost of manufacture and repair (due to the requirement to evacuate and bake out the detector and to bond, rather than screw, the window down). Re-filling a sealed-gas detector can cost over half the cost for a new detector.
Gas flow proportional counters do not have these problems and offer the best sensitivity to beta radiation, combined with a relatively low sensitivity to background gamma radiation. Firstly, the window is thin, so low-energy beta radiation is not cut out and may therefore be detected. Secondly, the possibility of photon interaction of (higher-energy) gamma rays in the fill gas in gas flow proportional counters is relatively low.
Having said that, gas flow proportional counters require a continuous flow of fill gas. The fill gas is typically a mixture of argon and methane (either P10, which is 90% (by volume) Ar and 10% CH4, or P7.5, which is 92.5% Ar and 7.5% CH4) or argon and carbon dioxide, stored in high-pressure cylinders. The physical size of the cylinders and the fact that they are high-pressure and may contain a flammable substance represent health and safety concerns, especially in nuclear installations. Furthermore, it can be difficult to obtain such cylinders, or alternatively to produce the fill gas mixture sufficiently purely, in some countries in the developing world.
It would therefore be desirable to provide an alternative or improved beta radiation monitor.